Technical Field
The present disclosure generally relates to radio frequency communications and, more specifically, to near-field communications based on a short-distance electromagnetic coupling between a terminal and a transponder. The present disclosure more specifically relates to the frequency adaptation of an oscillating circuit of a terminal.
Discussion of the Related Art
Electromagnetic transponder systems used in near field communications are now well known. They can, for example, be found in portable devices (Smartphones, touch pads, etc.), which are equipped with near field communication (NFC) devices. The operation of such systems is based on the transmission of a radio frequency radiation by a terminal or reader in order to communicate with, and possibly to remotely supply, a transponder present in the field of the terminal. The terminal and the transponder are each equipped with an oscillating circuit (antenna plus capacitive element) and are generally tuned to the same frequency (typically 13.56 MHz for the NFC standard).
An issue lies in the fact that the components used to form the oscillating circuits undergo drifts due not only to manufacturing tolerances, but also to temperature variations.
This problem is particularly critical for terminals which have to generate the electromagnetic field. Indeed, integrated circuit technology limits to a few volts the excursion of the voltage that can be provided. To generate a higher voltage, monolithic power components would have to be used, which is not desirable. As a consequence, the resonance should be generated outside of the integrated circuit to be able to reach an excursion of some ten volts (typically on the order of 30 volts). As a result, capacitive elements cannot be integrated. Now, with discrete component, dispersions generally reach approximately 10% and at best approximately 5%. Such tolerances are reflected on the resonance frequency.
Currently, terminals are most often formed with adjustable capacitive elements. The capacitive element setting is performed in a test or maintenance operation. This, however, does not enable to compensate for possible operating drifts linked, for example, to temperature variations.
A detuning between the oscillating circuits of the transponder and of the terminal is particularly important since the tuning conditions the quality of the transmission, and especially of the distance at which a transponder must be from a terminal for a communication to be able to occur.